1. Field of the Invention
This invention relates to a split-type multi-cylinder internal combustion engine operable on less than all of its cylinders under low load conditions but on all of the cylinders when the engine load exceeds a predetermined value.
2. Description of the Invention
It is generally known that internal combustion engines exhibit better fuel combustion and thus higher fuel economy when running under higher load conditions. In view of this fact, split type internal combustion engines have already been proposed which operate on less than all of the cylinders under low load conditions and on all of the cylinders when the engine load exceeds a given value. That is, under low load conditions, some of the cylinders are held inactive so that the other active cylinders can operate with relatively high loads. This is effective to achieve high fuel economy.
One difficulty with such split-type internal combustion engines is that during a split engine operation, air is discharged from the inactive cylinders to the exhaust system of the engine to cause a reduction in the temperature of the exhaust gases flowing through the catalyzer provided in the exhaust systems to thereby spoil its exhaust emission purifying performance.
In order to eliminate this disadvantage, an improved split-type internal combustion engine has been provided which has its intake passage bifurcated, downstream of the throttle valve, into first and second branches, the first branch leading to the active cylinders and the second branch leading to the inactive cylinders. The second branch has therein an air stop valve adapted to close during a split engine operation. The exhaust passage of the engine is divided, upstream of the catalyzer, into first and second branches, the first branch leading to the active cylinders and the second branch leading to the inactive cylinders. The engine also has an exhaust gas recirculation (EGR) passage having its one end opening into the second intake passage branch and the other end opening into the second exhaust passage branch. The EGR passage has therein an EGR valve adapted to open during a split engine operation.
During a split engine operation, substantially all of the exhaust gases discharged from the inactive cylinders is recirculated thereinto. This is effective to maintain the catalyzer at a high temperature conductive to its maximum performance and to reduce pumping losses in the inactive cylinders.
With such a conventional split engine, however, there is the possibility of escape of exhaust gases from the second intake passage branch to the first intake passage branch during a split engine operation due to a great pressure differential occurring across the air stop valve during a split engine operation. This results in imcomplete fuel combustion in the active cylinders.